The present invention relates to an optical phase plate and a liquid crystal display using same.
Recently, in the optical field and the optoelectronics field, a phase plate for controlling the phase of light has become one of important optical elements. For example, it is utilized as a 1/2 wavelength plate, a 1/4 wavelength plate, or a compensator for liquid crystal display. Particularly, in the field of liquid crystal display (LCD), the phase plate is now an essential optical element for improving the display quality. Above all, a super twisted nematic (STN) liquid crystal display is of a multiplex driven dot-matrix type which permits a large-screen display, and as compared with the conventional twisted nematic (TN) type liquid crystal display, the STN liquid crystal display is characteristic in that the contrast is high and the field angle is wide. Because of these characteristics the STN liquid crystal display is widely used in the field of liquid crystal display which requires a large-screen display such as, for example, personal computers, word processors and various data terminals. However, the STN type basically cannot make display in a black-and-white mode and it is unavoidable that the display is in green to yellowish red in the absence of voltage and in blue in the presence of voltage. Such a colored mode display is not only undesirable to the users but also involves a serious drawback of being unable to cope with coloring of a display unit. Various proposals have been made for eliminating this drawback. According to a typical method, a phase difference created when light passes through a liquid crystal display is compensated for to realize a black-and-white display, using an optical phase film which is a uni- or biaxially stretched polymeric film such as polycarbonate or polyvinyl alcohol film. In the production of such phase film for STN-LCD, however, it is necessary to obtain a film having a value of highly uniform retardation (birefringence X film thickness) over a large area, but this is extremely difficult industrially. Poor yield and high cost will result.
In the case of the above stretched film there also arises the problem of deteriorated contrast due to the fact that the dependence of the compensator birefringence upon wavelength is smaller than that of the liquid crystal birefringence. This is because at a certain specific wavelength the compensation is effected completely, while at another wavelength the compensation is incomplete, thus causing the decrease of dark level.
Also in such optical elements as 1/2 wavelength plate and 1/4 wavelength plate, it is difficult to control their phase difference accurately, and high cost will result.
It is an object of the present invention to overcome the above-mentioned problems of the prior art.
It is another object of the present invention to provide a phase plate easy to be manufactured and capable of controlling the phase difference thereof accurately.
It is a further object of the present invention to provide a liquid crystal display capable of making a black-and-white display at a wide field angle and a high contrast.